From ancient tools to modern skyscrapers and automobiles, steel has driven human innovation for hundreds of years. Abundant in the Earth's crust, iron and its associated alloys have provided humanity with solutions to many daunting developmental barriers. From humble beginnings, steel development has progressed considerably within the past two centuries, with new varieties of steel becoming available every few years. These steel alloys can be broken up into three classes based upon measured properties, in particular maximum tensile strain and tensile stress prior to failure. These three classes are: Low Strength Steels (LSS), High Strength Steels (HSS), and Advanced High Strength Steels (AHSS). Low Strength Steels (LSS) are generally classified as exhibiting tensile strengths less than 270 MPa and include such types as interstitial free and mild steels. High-Strength Steels (HSS) are classified as exhibiting tensile strengths from 270 to 700 MPa and include such types as high strength low alloy, high strength interstitial free and bake hardenable steels. Advanced High-Strength Steels (AHSS) steels are classified by tensile strengths greater than 700 MPa and include such types as Martensitic steels (MS), Dual Phase (DP) steels, Transformation Induced Plasticity (TRIP) steels, and Complex Phase (CP) steels. As the strength level increases the trend in maximum tensile elongation (ductility) of the steel is negative, with decreasing elongation at high tensile strengths. For example, tensile elongation of LSS, HSS and AHSS ranges from 25% to 55%, 10% to 45%, and 4% to 30%, respectively.
Production of steel continues to increase, with a current US production around 100 million tons per year with an estimated value of $75 billion. Steel utilization in vehicles is also high, with advanced high strength steels (AHSS) currently at 17% and forecast to grow by 300% in the coming years [American Iron and Steel Institute. (2013). Profile 2013. Washington, D.C.]. With current market trends and governmental regulations pushing towards higher efficiency in vehicles, AHSS are increasingly being pursued for their ability to provide high strength to mass ratio. The high strength of AHSS allows for a designer to reduce the thickness of a finished part while still maintaining comparable or improved mechanical properties. In reducing the thickness of a part, less mass is needed to attain the same or better mechanical properties for the vehicle thereby improving vehicle fuel efficiency. This allows the designer to improve the fuel efficiency of a vehicle while not compromising on safety.
One key attribute for next generation steels is formability. Formability is the ability of a material to be made into a particular geometry without cracking, rupturing or otherwise undergoing failure. High formability steel provides benefit to a part designer by allowing for the creation of more complex part geometries allowing for reduction in weight. Formability may be further broken into two distinct forms: edge formability and bulk formability. Edge formability is the ability for an edge to be formed into a certain shape. Edges on materials are created through a variety of methods in industrial processes, including but not limited to punching, shearing, piercing, stamping, perforating, cutting, or cropping. Furthermore, the devices used to create these edges are as diverse as the methods, including but not limited to various types of mechanical presses, hydraulic presses, and/or electromagnetic presses. Depending upon the application and material undergoing the operation, the range of speeds for edge creation is also widely varying, with speeds as low as 0.25 mm/s and as high as 3700 mm/s. The wide variety of edge forming methods, devices, and speeds results in a myriad of different edge conditions in use commercially today.
Edges, being free surfaces, are dominated by defects such as cracks or structural changes in the sheet resulting from the creation of the sheet edge. These defects adversely affect the edge formability during forming operations, leading to a decrease in effective ductility at the edge. Bulk formability on the other hand is dominated by the intrinsic ductility, structure, and associated stress state of the metal during the forming operation. Bulk formability is affected primarily by available deformation mechanisms such as dislocations, twinning, and phase transformations. Bulk formability is maximized when these available deformation mechanisms are saturated within the material, with improved bulk formability resulting from an increased number and availability of these mechanisms.
Edge formability can be measured through hole expansion measurements, whereby a hole is made in a sheet and that hole is expanded by means of a conical punch. Previous studies have shown that conventional AHSS materials suffer from reduced edge formability compared with other LSS and HSS when measured by hole expansion [M. S. Billur, T. Altan, “Challenges in forming advanced high strength steels”, Proceedings of New Developments in Sheet Metal Forming, pp. 285-304, 2012]. For example, Dual Phase (DP) steels with ultimate tensile strength of 780 MPa achieve less than 20% hole expansion, whereas Interstitial Free steels (IF) with ultimate tensile strength of approximately 400 MPa achieve around 100% hole expansion ratio. This reduced edge formability complicates adoption of AHSS in automotive applications, despite possessing desirable bulk formability.